Experiments are proposed in two areas relating to the stimulus properties of Drugs. Project 1 will investigate whether centrally acting drugs can become conditioned stimuli in classical conditioning paradigms, thus acquiring conditioned reactions that may augment or interfere with the usual effects of the drugs. Five experiments are proposed to determine the ease of formation of such conditioned reactions to drug conditioned stimuli, the variety of situations in which they occur, and the rapidity with which they extinguish. Project 2 will test for the occurrence of stimulus masking in the drug discrimination preparation to determine whether this is a frequent or rare phenomenon. The results will indicate whether masking is a serious source of artefact in normal applications of the drug discrimination procedure. A total of 6 separate experiments are proposed.